fix(ngScenario): select(...).option(val) should select exact value match

This pull requests modifies ngScenario's `select(...).option(val)` such that it closes #2855.

This fix prefers exact matches when available, otherwise it reverts to the previous 'contains' behaviour for backwards compatibility.

This pull request also updates the dslSpec tests to ensure proper behaviour going forward for both the exact text match and the :contains text match.